1. Field of the Invention
This invention generally relates to an apparatus and method for adjusting the scan line location of a light beam on a photoreceptor. More particularly, this invention relates to multi-layer image registration in single or multipass color printers.
2. Description of Related Art
Flying spot scanners (often referred to as raster output scanners or ROSs) conventionally have a reflective multifaceted polygon mirror that is rotated about its central axis to repeatedly sweep one or more intensity modulated beams of light across a photosensitive recording medium in a line scanning direction (known as the fast-scan direction) while the recording medium is being advanced in an orthogonal or "process" direction (known as the slow scan direction), such that the beam scans the recording medium in accordance with a raster scanning pattern. Digital printing is performed by serially intensity modulating each of the beams in accordance with the binary sample string, whereby the recording medium is exposed to the image represented by the samples as it is being scanned. Printers that sweep several beams simultaneously are referred to as multi-beam printers. Both ROS and multi-beam printer techniques are illustrated in U.S. Pat. No. 4,474,422 to Kitamura, the subject matter of which is incorporated herein by reference.
High speed process color or multi-highlight color xerographic image output terminals require multiple independently addressable raster lines to be printed simultaneously at separate exposure stations. This is called multi-station printing. Conventional architectures from multi-station process color printers use a plurality of separate ROSs, usually four independent ROSs, one for each system color, for example, as illustrated in U.S. Pat. Nos. 4,847,642 and 4,903,067 to Murayama et al., the disclosures of which are incorporated herein by reference.
One problem with these systems are the high cost related to the multiple ROSs, the high cost of producing nearly identical multiple ROSs and associated optics, and the difficulty of registering the system colors.
U.S. Pat. No. 5,243,359 to Tibor Fisli, the disclosure of which is incorporated herein by reference, discloses a ROS system suitable for deflecting multiple laser beams in a multi-station printer. In U.S. Pat. No. 5,243,359, a rotating polygon mirror simultaneously deflects a plurality of clustered, dissimilar wavelength laser beams, having their largest divergent angles parallel to one another. The laser beams are subsequently separated by a plurality of optical filters and are directed to their associated photoreceptors. Similarly dimensioned spots are obtained on each photoreceptor by establishing similar optical path lengths for each beam. The laser diodes in U.S. Pat. No. 5,243,359 are arranged in the slow scan direction (i.e., sagittally offset). Diodes arranged in the slow scan direction must be arranged such that they are packed closely in a direction parallel to the polygon mirror's rotational axis to minimize beam characteristic deviations such as spot size, energy uniformity, bow and linearity. Thus, the laser diodes are kept as closely as possible in the direction parallel to the polygon mirror's rotational axis so that the light beams strike nearly the same portion of the polygon mirror as possible.
U.S. Pat. No. 5,341,158 to James Appel et al., the disclosure of which is incorporated herein by reference, discloses a ROS system in which the laser beams are arranged in the fast scan direction (i.e., tangentially offset). Wavelength discriminating optics are used to alternately separate and reflect the light beams from a polygon mirror.
Additionally, U.S. application Ser. No. 07/948,530 to James Appel et al., filed Sep. 22, 1992, the disclosure of which is incorporated herein by reference, discloses one system that separates laser beams based on both the wavelength and the polarization of each of the beams.
The ROS systems described in U.S. Pat. No. 5,243,359 and 5,341,158 are generally described with respect to single spot scanning whereby only a single light beam is directed onto each respective photoreceptor. However, it is understood that these systems can be modified to incorporate multispot printing whereby more than one light beam is directed onto each respective photoreceptor.
Polygon scanners such as that described above are well known in the art and are described, for example, in "Laser Scanning For Electronic Printing," Proceedings of the IEEE, Vol. 70, No. 6, June 1982 by John C. Urbach et al., the disclosure of which is incorporated by reference. Other optical polygon scanner embodiments are: similarly known and are within the scope of this invention.
In both single spot and multispot imaging using a plurality of photoreceptors, it is necessary to strictly control the exact placement of the light beam on each of the photoreceptors. This process is called registration. Registration is a very serious problem that requires strict control of the light beams being directed onto the photoreceptors. As is well known, the precise registration of the toner layers (such as yellow, cyan, magenta and black) is essential to produce high quality color images. The misregistration of one of the four light beams can cause serious consequences that are visually noticeable to the user.